This 30-minute webinar enables participants to understand the importance of wall bracing to resist wind and earthquake loads, recognize the role of portal frames in resisting lateral loads, identify the different portal frame options in the International Residential Code (IRC), spot the key elements of a portal frame and avoid common errors. This webinar does not offer continuing education credits at this time.
IRC Table 602.10.5 – “Minimum Length of Braced Wall Panels” indicates the minimum length of braced wall panel required for all bracing methods, including the three portal frame methods. The minimum length for PFG is 24 inches if the portal header height is eight feet or less.
A portal frame header can be positioned at the top of the wall as long as it meets the wall and portal frame header height restrictions and the minimum braced wall length requirements in IRC Table R602.10.5. It also must be constructed according to the applicable portal frame detail in the IRC. This would include a king stud that runs continuous along the outside of the portal frame to the top of the header.
The tension straps required per the IRC are prescriptive, but an alternate solution may be engineered.
One alternate solution to the manufactured steel strap is a 2x4 or 2x6 lumber strap. The 2x lumber alternate must have a tensile capacity of about 4200 lbs.
The 2x should be fastened to the jack stud closest to the opening and extend up over the header and the pony wall, if there is one.
At the 1,000 lb. minimum strap requirement, the 2x4 or 2x6 must be attached with 14—10d common nails (0.148" diameter). Seven nails should be placed in the header region and seven nails should be placed in the wall framing below the header segment. If strap capacity requirement is greater than 1,000 lb. the lumber tension capacity should be checked and nailing increased.
Other engineering alternatives are also possible, including a 2x10.
The IRC prescriptive bracing portal frame methods are limited to a maximum 10-foot portal frame header height and a maximum 12-foot high wall, if a pony wall is used. Pony walls above the header cannot exceed four feet.
A higher header height could be accomplished prescriptively by using a higher concrete stem wall. The 10' maximum header height is measured from the top of the concrete stem wall. The stem wall height cannot exceed 48".
See Section R602.10.9 Item 4 and Figure R602.10.9 for reinforcement requirements for foundation walls. In addition, Method PFH requires (1)—#4 horizontal bar at top and bottom of footing. See Figure R602.10.6.2.
Any other configuration for a portal frame will require engineering analysis and will not fall under the scope of the IRC. Manufacturers of proprietary narrow wall products should also be checked as possible solutions for such designs.
APA conducted cyclic testing to establish portal frames with hold-downs and portal frames without hold-downs. Based on the extensive testing at the APA research laboratory, APA drafted proposals to add these portal-frame designs to the IBC and IRC as bracing alternates.
For guidance on engineered portal frames, APA Technical Topic TT-100, A Portal Frame with Hold-Downs for Wall Bracing or Engineered Applications can be downloaded here.
Longer nails are not required when a header is made up of two members, provided the members are mechanically connected according to IRC Table R602.3(1), or according to the header manufacturer’s installation instructions (when two or more plies of LVL or other structural composite lumber are used). This enables the members to act as one unit.
The IRC limits the maximum finished garage door opening width to 18 feet for all three portal frame methods. This information is noted on IRC Figures 602.10.6.2, 602.10.6.3, and 602.10.6.4.
For a larger garage, multiple portal frames can be used. For example: a four-car garage with two double car openings, a three-car garage with a single car opening and a double car opening, multiple single car openings. These can all use combinations of single or double portal frames.
Keep in mind that method CS-PF is currently limited to four portal frames in a single braced wall line. (Note that this limitation will not be in the 2021 IRC, due to a successful IRC code change proposal based on testing at the APA research lab.)
You may be looking at the CS-WSP, CS-SFB section of IRC Table R602.10.5. In the 2015 IRC, the information for the three portal frame methods is directly above that section and the minimum length is based on the wall height.
In the 2018 IRC, the information for the three portal frame methods have been moved to the bottom of the table and the minimum length is based on the header height.
A 12" deep footing is required under the opening for method PFH. If the footing is not poured properly, the engineer of record should provide a repair.
Method PFH provides 48" of braced wall panel for each side of the opening where there is a portal frame. If an opening has a single PFH, it provides 48" of braced wall panel. A double PFH would provide 96" of braced wall panel.
See Table R602.10.5 for the minimum length of braced wall panels and the contributing length.
When we talk about a portal frame contributing 1.5x actual length, we are talking about the linear footage of braced wall panel that the portal frame is contributing to the required amount of bracing for the braced wall line.
IRC Section R602.10.5.1 and Table R602.10.5 provide this information for each of the bracing methods in the IRC.
“1.5x actual length” means that the portal frame provides an amount of bracing equal to 1.5 times the length of the portal frame panel. For example, a 24" wide portal frame would contribute 36" of bracing towards the required amount of bracing for that braced wall line .
The purpose of the tension strap is to prevent “hinging” of the header or pony wall when the wind blows perpendicular to the wall containing the portal frame.
Additional portal frame detailing on the inside of the wall will not prevent the header or pony wall from hinging. It would only provide additional lateral support and is not addressed in the IRC.
A tension strap, or a member with the required tension capacity per IRC Table 602.10.6.4, is necessary to prevent hinging.
Methods PFH and PFG can only be used in single story structures or on the first floor of a two-story structure. Both methods can be used at a garage door opening when there is a story above the garage.
Method CS-PF can be used on the first floor or second floor of a structure. This method can be used at a garage door opening and also around an opening on the second floor of a structure, including above the garage. See Figure R602.10.6.4.
There is no field repair for a header that isn’t continuous over the wall panel. A header of the correct length must be installed so that the portal frame can achieve the rigidity required to resist the lateral loads applied to the structure.
CS-PF must be used with CS-WSP no matter the seismic zone.
IRC Section R602.10.4.2 explains the requirements for the continuously sheathed methods, including use of CS-PF in seismic design category D and above.
Blocking is only required if there is a joint in the panels. If one continuous panel is installed, no additional blocking is required.
The king stud helps to stabilize the header and prevent the wall from hinging under a wind load perpendicular to the opening. It also works with the header and other portal frame elements to create a semi-rigid frame that enables the portal frame to resist lateral loads.
Per IRC Figures R602.10.6.2 and R602.10.6.4, the king stud is fastened to the header with six—16d sinker nails for methods PFH and CS-PF. For method PFG, IRC Figure R602.10.6.3 does not indicate the nailing requirement for the king stud to the header, but six—16d sinker nails can be assumed to also apply to this method.
The portal frame header should be sized for gravity loads, but also must meet the minimum size of 3 x 11-1/4" required per the IRC portal frame details. The opening size is also limited to a maximum of 18 feet. With these limitations, a portal frame header should be able to resist gravity loads in addition to the expected lateral loads.
The details for the portal frame methods in IRC Figures 602.10.6.2, 602.10.6.3, and 602.10.6.4 require a 2"x2"x3/16" plate washer on all anchor bolts. These details apply to all seismic zones.
The header of the double portal frame must extend over the length of the center pier, with a jack stud and king stud on the end. The header for the single portal frame will extend just past the opening and will be supported by a separate jack stud and king stud.
The wood structural panel at the center pier will extend to cover the jack stud and king stud of the single portal frame; however, the length of the panel for the double portal frame will be the distance from the edge of the opening to the edge of the king stud supporting the double portal frame header. The length of panel that extends over the jack stud and king stud for the single portal frame does not count towards the length of panel for the double portal frame.
If standard 2x studs are used for the jack studs and king studs, the “break” between the two portal frames will be 3" from the edge of the opening on the single portal frame.
One way to accomplish this may be to construct one of the portal frames on the ground and then tilt it up into place. This would allow the nailing into the end of the king studs.
For a concrete foundation, the detailing is the same, regardless of the height of the foundation wall, as long as the portal frame does not exceed the allowable values in IRC Table 602.10.5 and the foundation wall is reinforced according to Section R602.10.9 Item 4. In addition, Method PFH requires (1)—#4 horizontal bar at top and bottom of footing. See Figure R602.10.6.2.
For a masonry foundation, the detailing is the same, regardless of the height of the foundation wall, as long as the portal frame does not exceed the allowable values in IRC Table 602.10.5 and the foundation wall is built according to Section R602.10.9 Item 3 and Figure R602.10.9, which limits the height of the masonry foundation wall to 48".
Note that Method PFH is not permitted to attach to masonry stem walls per this section.
The portal frame methods in the IRC were not tested for a condition with a header continuous over a center panel, between two openings. If a home is designed by a licensed engineer in this manner, it can be assumed that this condition has been properly analyzed and is structurally sound. The designer should also take into consideration the increased buckling risk of a continuous header that spans multiple garage openings, particularly at intermediate walls or columns that are not sufficiently laterally braced.
For a home designed according to the prescriptive methods of the IRC, the header must not run continuous over the center support if it is to be in compliance with the code.
Method PFH in the IRC was only tested using hold-downs that are embedded in the concrete. This is the only condition that will meet the prescriptive requirements of the IRC. If the hold-downs are not embedded in the concrete per IRC Figure R602.10.6.2 and an alternative anchorage is required, the design of that alternative anchorage will need to be performed by a licensed engineer, since it will no longer qualify as a prescriptive method.
The IBC and IRC PFH (with hold-downs) bracing methods require embedded strap-type hold-downs. HD-type or other non-strap hold-down types do not meet this requirement. In consultations with Simpson Strong-Tie, we’ve been told that a strap-type hold-down will have considerably less deflection than an HD type. Using an HD-type hold-down would increase the deflection of the portal frame, and it may not work in conjunction with the other lateral force resisting elements in the system.